By NICHOLAS WADE, NYTimes
A team of geneticists studying the ancestry of Jewish communities has found an unusual genetic signature that occurs in more than half the Levites of Ashkenazi descent. The signature is thought to have originated in Central Asia, not the Near East, which is the ancestral home of Jews. The finding raises the question of how the signature became so widespread among the Levites, an ancient caste of hereditary Jewish priests.
The genetic signature occurs on the male or Y chromosome and comes from a few men, or perhaps a single ancestor, who lived about 1,000 years ago, just as the Ashkenazim were beginning to be established in Europe. Ashkenazim, from whom most American Jews descend, are one of the two main branches of Jews, the other being the Sephardim, whose ancestors were expelled from Spain.
The new report, published in the current issue of the American Journal of Human Genetics, was prepared by population geneticists in Israel, the United States and England, who have been studying the genetics of Jewish communities for the last six years.
They say that 52 percent of Levites of Ashkenazi origin have a particular genetic signature that originated in Central Asia, although it is also found less frequently in the Middle East. The ancestor who introduced it into the Ashkenazi Levites could perhaps have been from the Khazars, a Turkic tribe whose king converted to Judaism in the eighth or ninth century, the researchers suggest.
Their reasoning is that the signature, a set of DNA variations known as R1a1, is common in the region north of Georgia that was once occupied by the Khazar kingdom. The signature did reach the Near East, probably before the founding of the Jewish community, but it is still rare there. The scholars say they cannot exclude the possibility that a Jewish founder brought the signature on his Y chromosome to the Ashkenazi population, but they consider that a less likely explanation.
The present descendants of the Khazars have not been identified. Dr. Michael Hammer of the University of Arizona, one of the authors of the report, said he was looking among the Chuvash, a Turkic-speaking people of the Volga Valley, to see if they might have contributed the R1a1 signature.
Dr. Shaye Cohen, professor of Hebrew literature and philosophy at Harvard University, said he could see no problem with outsiders being converted to the Jewish community. He said he considered it less probable, however, that outsiders would become Levites, let alone founding members of the Levite community in Europe. The connection with the Khazars is "all hypothesis," he said.
Even if the Khazar hypothesis is correct, it would have no practical effect on who is a Levite today. "Genetics is not a reality under rabbinic law," Dr. Cohen said. "Second, the function of Levites is so minimal it doesn't mean anything."
Six years ago Dr. Hammer and Dr. Karl Skorecki, of the Technion and Rambam Medical Center in Haifa, looked at the Y chromosomes of both Levites and Cohanim. Both are hereditary priesthoods passed from father to son. They were important in ancient Israel, but sometime between 200 B.C. and A.D. 500 their functions were taken over by rabbis, and Jewish status came to be defined by the biologically more reliable standard of maternal descent.
If the patrilineal descent of the two priestly castes had indeed been followed as tradition describes, then all Cohanim should be descended from Aaron, the brother of Moses, and all Levites from Levi, the third son of the patriarch Jacob. Dr. Hammer and Dr. Skorecki found that more than half the Cohanim, in both the Ashkenazi and Sephardi communities, did indeed carry the same genetic signature on their Y chromosome. Their ancestor lived some 3,000 years ago, based on genetic calculations, and may indeed have been Aaron, Dr. Skorecki said.
But the picture among the Levites was less clear, suggesting that they had a mixed ancestry. Dr. Hammer and Dr. Skorecki returned to the puzzle for their new report, based on data gathered from nearly 1,000 men of Ashkenazi and Sephardi origin and neighboring non-Jewish populations.
They found that the dominant signature among the Levites was the R1a1 signature, which is different from the Cohanim signature. The paternal ancestry of the Ashkenazi and Sephardic Levites is different, unlike the Cohanim from the two branches, who resemble each other and presumably originated before the two branches split. And the ancestor of the R1a1 signature apparently lived 2,000 years more recently than the founder of the Cohanim signature.
The Levites' pedigree does not seem to accord with tradition as well as the Cohanim one does but is venerable nonetheless. "How many people can trace their ancestry back to the 17th century, let alone a thousand years?" Dr. Hammer said.
Saturday, 11 March 2006
By NICHOLAS WADE, NYTimes
Posted @ 17:29